Heat-sensitive imaging material for making lithographic printing plates requiring no processing

ABSTRACT

According to the present invention there is provided a heat-sensitive imaging material for making lithographic printing plates which require no processing. The heat-sensitive imaging element comprises on a lithographic base having a hydrophilic surface a metallic or metal oxide layer and on top thereof an oleophobic  oleophilic polymeric layer having a thickness of less than 5 μm and comprising a polymer containing phenolic groups.

This application claims benefit of Provisional Application Ser. No.60/079,871 filed Mar. 30, 1998.

FIELD OF THE INVENTION

The present invention relates to a heat-sensitive imaging element formaking lithographic printing plates. More specifically the inventionrelates to a heat-sensitive imaging element which requires noprocessing.

BACKGROUND OF THE INVENTION

Lithographic printing is the process of printing from specificallyprepared surfaces, some areas of which are capable of accepting ink,whereas other areas will not accept ink.

In the art of photolithography, a photographic material is madeimagewise receptive to oily or greasy ink in the photo-exposed (negativeworking) or in the non-exposed areas (positive working) on aink-repelling background.

In the production of common lithographic plates, also called surfacelitho plates or planographic printing plates, a support that hasaffinity to water or obtains such affinity by chemical treatment iscoated with a thin layer of a photosensitive composition. Coatings forthat purpose include light-sensitive polymer layers containing diazocompounds, dichromate-sensitized hydrophilic colloids and a largevariety of synthetic photopolymers. Particularly diazo-sensitizedsystems are widely used.

Upon imagewise exposure of such light-sensitive layer the exposed imageareas become insoluble and the unexposed areas remain soluble. The plateis then developed with a suitable liquid to remove the diazonium salt ordiazo resin in the unexposed areas.

On the other hand, methods are known for making printing platesinvolving the use of imaging elements that are heat-sensitive ratherthan photosensitive. A particular disadvantage of photosensitive imagingelements such as described above for making a printing plate is thatthey have to be shielded from the light. Furthermore they have a problemof stability of sensitivity in view of the storage time and they show alower resolution. The trend towards heat-sensitive printing plateprecursors is clearly seen on the market.

EP-A-444 786, JP-63-208036, and JP-63-274592 disclose photopolymerresists that are sensitized to the near IR. So far, none has provedcommercially viable and all require wet development to wash off theunexposed regions. EP-A-514 145 describes a laser addressed plate inwhich heat generated by the laser exposure causes particles in the platecoating to melt and coalescence and hence change their solubilitycharacteristics. Once again, wet development is required.

EP-A-652 483 discloses a lithographic printing plate requiring nodissolution processing which comprises a substrate bearing aheat-sensitive coating, which coating becomes relatively morehydrophilic under the action of hat. Said system yields a positiveworking printing plate. EP-A-609 941 describes a heat-mode recordingmaterial comprising on a substrate a metallic layer and a thinhydrophobic layer which becomes hydrophilic upon exposure. However thelithographic performance of the obtained printing plate is poor.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a heat-sensitiveimaging element for preparing lithographic printing plates requiring nodissolution processing and having a high lithographic performance (inkacceptance, scratch resistance, durability)

SUMMARY OF THE INVENTION

According to the present invention there is provided a heat-sensitiveimaging element for making lithographic printing plates comprising on alithographic base, having a hydrophilic surface, a metallic layer ormetal oxide layer and on top thereof an oleophobic oleophilic polymerlayer having a thickness of less than 5 μm characterised in that saidpolymer layer comprises a polymer containing phenolic groups.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that according to the present invention, using animaging element as described above, lithographic printing platesrequiring no processing and having an excellent lithographic performancecan be obtained.

Metallic layers or metal oxide layers suitable for use in accordancewith the invention comprise metals or metal oxides converting theactinic radiation to heat so that the oleophobicity oleophilicity of theoleophobic oleophilic top-layer is destroyed. The thickness of themetallic layer or metal oxide layer is preferably from 0.01 μm to 2 μm,and most preferably from 0.05 μm to 1.5 μm. Specific examples of metallayers or metal oxide layers are aluminum, titanium oxide, bismuth andsilver of which the three latter are preferred.

A silver layer for use in this invention as the metallic layer can bemade according to the principles of the silver complex diffusiontransfer reversal process, hereinafter called DTR-process, having beendescribed, e.g. in U.S. Pat. No. 2,352,014 and in the book “PhotographicSilver Halide Diffusion Processes” by André Rott and Edith Weyde—TheFocal Press—London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wiseexposed photographic silver halide emulsion layer material istransformed with a so-called silver halide solvent into soluble silvercomplex compounds which are allowed to diffuse into an image-receivingelement and are reduced therein with a developing agent, generally inthe presence of physical development nuclei, to form a silver imagehaving reversed image density values (‘DTR-image’) with respect to theblack silver image obtained in the exposed areas of the photographicmaterial.

In another method for providing a metal layer on the lithographic basehaving a hydrophilic surface a silver halide emulsion disposed on alithographic base having a hydrophilic surface is strongly exposed toactinic radiation and then developed, or otherwise processed to maximumblackness. The black opaque emulsion is converted to a reflectiverecording material by heating at least to 270° C. in an oxygencontaining environment until the emulsion coating assumes a shinyreflective appearance. Such method is disclosed in U.S. Pat. No.4,314,260.

According to an alternative method for providing a metal layer on thelithographic base the metal is provided using vapour or vacuumdeposition.

According to another embodiment of the invention the metallic layer canbe a bismuth layer that can be provided by vacuum deposition.

A drawback of the method of preparation of a thin bismuth recordinglayer by vacuum deposition is the fact that this is a complicated,cumbersome and expensive process.

Therefore, in EP-A-97201282 the vacuum deposition is replaced by coatingfrom an aqueous medium. According to this disclosure a thin metal layeris formed by the following steps:

(1) preparing an aqueous medium containing ions of a metal,

(2) reducing said metal ions by a reducing agent thus forming metalparticles,

(3) coating said aqueous medium containing said metal particles on atransparent support.

As a metal oxide layer preferably a titanium oxide layer is used. Thislayer can be applied to the substrate by vacuum deposition,electron-beam evaporation or sputtering.

The oleophobic oleophilic layer provided on top of the metallic layer ormetal oxide layer comprises a polymer containing phenolic groups.Preferred polymers containing phenolic groups are phenolic resins (e.g.novolac) or hydroxyphenyl substituted polymers (e.g.polyhydroxystyrenes). The oleophobic oleophilic layer has a thickness ofless than 5 μm. As a consequence a highly sensitive heat-sensitiveimaging element is obtained. The use of a polymer containing phenolicgroups furthermore improves the lithographic performance (inkacceptance, scratch resistance, durability) of the lithographic printingplates obtained according to the present invention.

According to one embodiment of the present invention, the lithographicbase having a hydrophilic surface can be an anodised aluminum. Aparticularly preferred lithographic base having a hydrophilic surface isan electrochemically grained and anodised aluminum support. Mostpreferably said aluminum support is grained in nitric acid, yieldingimaging elements with a higher sensitivity. According to the presentinvention, an anodised aluminum support may be treated to improve thehydrophilic properties of its surface. For example, the aluminum supportmay be silicated by treating its surface with a sodium silicate solutionat elevated temperature, e.g. 95° C. Alternatively, a phosphatetreatment may be applied which involves treating the aluminum oxidesurface with a phosphate solution that may further contain an inorganicfluoride. Further, the aluminum oxide surface may be rinsed with acitric acid or citrate solution. This treatment may be carried out atroom temperature or can be carried out at a slightly elevatedtemperature of about 30 to 50° C. A further increasing treatmentinvolves rinsing the aluminum oxide surface with a bicarbonate solution.Still further, the aluminum oxide surface may be treated withpolyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoricacid esters of polyvinyl alcohol, polyvinylsulphonic acid,polyvinylbenzenesulphonic acid, sulphuric acid esters of polyvinylalcohol, and acetals of polyvinyl alcohols formed by reaction with asulphonated aliphatic aldehyde. It is further evident that one or moreof these post treatments may be carried out alone or in combination.

According to another embodiment in connection with the presentinvention, the lithographic base can comprise a flexible support, suchas e.g. paper or plastic film, provided with a hardened hydrophiliclayer. A particularly suitable hardened rough hydrophilic layer may beobtained from a hydrophilic binder hardened with a hardening agent suchas formaldehyde, glyoxal, polyisocyanate or preferably a hydrolysedtetra-alkylorthosilicate.

As hydrophilic binder there may be used hydrophilic (co)polymers such asfor example, homopolymers and copolymers of vinyl alcohol, acrylamide,methylol acrylamide, methylol methacrylamide, acrylic acid, methacrylicacid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleicanhydride/vinylmethylether copolymers.

A hardened hydrophilic layer on a flexible support used in accordancewith the present embodiment preferably also contains substances thatincrease the mechanical strength and the porosity of the layer e.g.colloidal silica. In addition inert particles of larger size than thecolloidal silica can be added e.g. silica prepared according to Stoberas described in J. Colloid and Interface Sci., Vol. 26, 1968, pages 62to 69 or alumina particles or particles having an average diameter of atleast 100 nm which are particles of titanium dioxide or other heavymetal oxides. Incorporation of these particles gives the surface of thehardened hydrophilic layer a uniform rough texture consisting ofmicroscopic hills and valleys.

The thickness of the hardened hydrophilic layer may vary in the range of0.2 to 25 μm and is preferably 1 to 10 μm.

Particular examples of suitable hardened hydrophilic layers for use inaccordance with the present invention are disclosed in EP-A601 240,GB-P-1 419 512, FR-P-230 354, U.S. Pat. No. 3,971,660, U.S. Pat. No.4,284,705 and EP-A 514 490.

As support on which the hydrophilic layer is provided it is particularlypreferred to use a plastic film e.g. substrated polyethyleneterephthalate film, cellulose acetate film, polystyrene film,polycarbonate film etc . . . . The plastic film support may be opaque ortransparent.

It is particularly preferred to use a polyester film support to which anadhesion improving layer has been provided. Particularly suitableadhesion improving layers for use in accordance with the presentinvention comprise a hydrophilic binder and colloidal silica asdisclosed in EP-A-619 524, EP-A-620 502 and EP-A-619 525. Preferably,the amount of silica in the adhesion improving layer is between 200 mgper m² and 750 mg per m². Further, the ratio of silica to hydrophilicbinder is preferably more than 1 and the surface area of the colloidalsilica is preferably at least 300 m2 per gram, more preferably a surfacearea of 500 m² per gram.

In accordance to the method of the present invention for obtaining alithographic printing plate the heat-sensitive imaging element isimage-wise scanning exposed using a laser, preferably a laser thatoperates in the infrared or near-infrared, i.e. wavelength range of700-1500 nm. Most preferred are laser diodes emitting in thenear-infrared.

After the exposure the imaging element can be used without an additionalwet treatment as a lithographic printing plate.

The printing plate obtained according to the present invention can alsobe used in the printing process as a seamless sleeve printing plate.This cylindrical printing plate wich has as diameter the diameter of theprint cylinder is slided on the print cylinder instead of applying in aclassical way a classically formed printing plate. More details onsleeves are given in ‘Grafisch Nieuws’ ed. Keesing, 15, 1995, page 4 to6.

The following examples illustrate the present invention without limitingit thereto. All parts and percentages are by weight unless otherwisespecified.

EXAMPLES Example 1

Preparation of the DTR Material

On the back of a polyethylene terephthalate support with a thickness of175μ, was coated a layer from a 11% wt solution in demineralized water(pH=4), with a wet thickness of 50 μm. The resulting layer contained74.7% of titaniumdioxide, 8.6% of polyvinylalcohol, 16.2% of hydrolysedtetramethylorthosilicate and 0.5% wetting agents.

On the other side of the polyethylene terephthalate support which isprovided with a hydrophilic subbing layer, is first coated a layer froma 20% wt solution in demineralized water pH=4), with a wet coatingthickness of 50 μm. This layer contained 82.7% of titaniumdioxide, 9.1%of polyvinylalcohol, 8.2% of hydrolysed tetramethylorthosilicate and0.17% of palladiumsulphide (particle size 2-3 nm). On this base layer, alayer of palladiumsulphide particles (2-3 nm) is coated from a 0.24% wtsolution (pH=9) in demineralized water, with a wet thickness of 13 μm.Finally, an emulsion layer and top layer were simultaneously coated bymeans of the cascade coating technique. The emulsion layer was coatedwith a wet thickness of 30 μm and such that the silver halide coverageexpressed as AgNO₃ was 2.50 g/m² and the gelatin content was 1.50 g/m².The toplayer was coated with a wet thickness of 15 μm such that thegelatin content was 0.7 g/m². The top layer further contained 61 mg/m²of Levanyl Rot and 0.14 g/m² matting agent.

Preparation of the Heat-Sensitive Imaging Element

To obtain a heat-sensitive imaging element according to the presentinvention, the unexposed DTR material as described above was developedfor 12 s at 24° C. in an aqueous alkaline solution having the followingingredients:

Anhydrous sodium sulphite 120 g Sodium hydroxide 22 gCarboxymethylcellulose 4 g Potassium bromide 0.75 g Anhydrous sodiumthiosulphate 8 g Aluminum sulphate.18H₂O 8 g Ethylene diaminetetraacetic acid tetrasodium salt 4.2 g Hydroquinone 20 g Methylfenidon6.25 g Demineralized water to make 1 L pH (25° C.) > 12.5

The initiated diffusion transfer was allowed to continue for 18 s toform a silver layer, whereafter the material was rinsed with watercontaining 0.03% of trypsine at 50° C.

The thus obtained metallic silver layer was provided with a hydrophobiclayer by guiding the material through a finisher at 45° C., having thefollowing composition:

Dextran 70000 40 g Polyethyleneglycol 200 50 mlSodiumdihydrogenphosphate.2H₂O 20 g Citric acid 22 g Potassium nitrate12.5 g Sodium hydroxide 12.6 g 1-phenyl-5-mercaptotetrazole 0.5 gBiocide 0.1 g Wetting agent 261.5 mg Demineralized water to make 1 L pH(25° C.) = 5.95

Exposing the Heat-Sensitive Imaging Element

This material was imaged with:

1. a Gerber C42T™ internal drum platesetter at 275 m/s and 2540 dpi. Thepower level of the laser in the image plane was 5.4 W.

2. an Isomel diode external drum platesetter at 3.2 m/s and 5 3600 dpi.The power level in the image plane was 253 mW.

Printing the Imagwise Exposed Element

The plates were both printed on a Heidelberg GTO46 printing machine witha conventional ink (Van Son rubberbase) and fountain solution(Rotamatic), resulting in excellent prints without any scumming in theIR-exposed areas and good ink-uptake in the unexposed areas. Theprinting results with respect to image quality are presented in table 1.

TABLE 1 Dot rendering Dot rendering Laser (100^(th) print - 200 lpi) 40%50% 70% screen Internal drum 4-91 61 73 91 External drum 2-94 64 75 88

Example 2

Preparation of the DTR Material

The DTR material was prepared as described in example 1.

Preparation of the Heat-Sensitive Imaging Element

To obtain a heat-sensitive imaging element according to the presentinvention, the unexposed DTR material was developed for 12 s at 24° C.in an aqueous alkaline solution as described in example 1.

The initiated diffusion transfer was allowed to continue for 18 s toform a silver layer, whereafter the material was rinsed with water at50° C.

One of the thus obtained metallic silver layers was used as such, onewas coated with a polyethylene layer (2 g/m²) and a 3 th one was coatedwith a novolac layer (2 g/m² Alnovol SPN452).

A 4th material was prepared as described in example 1.

Exposing the Heat-Sensitive Imaging Element

The 4 materials were all imaged with a Gerber C42T™ internal drumplatesetter at 12,000 rpm (367 m/s, pixel dwell time 0.032 μs) and 2540dpi. The power level of the laser in the image plane was 5.4 w.

After imaging, the plates were printed without any additional wettreatment.

Printing the Imagewise Exposed Element

The plates were printed on a Heidelberg GTO46 printing machine undermore critical conditions than in example 1 with a conventional ink (K+E)and a fountain solution of 5% G67c (commercially available fromAgfa-Gevaert N.V.)+10% isopropanol.

The plate that did not get an extra coating layer after developing, didnot show any ink-uptake in the unexposed areas and the material preparedas described in example 1 showed a slower ink-uptake while the one withpolyethylene as a final coating, resulted in a better ink-uptake and noscumming, but the image was already damaged after 25 prints. Finally theone with the novolac coating on top, showed a good ink-uptake, noscumming and a run-length>3000 prints.

Example 3

Preparation of the DTR Material

The DTR material was prepared as described in example 1 or 2.

Preparation of the Heat-Sensitive Imaging Element

To obtain a heat-sensitive imaging element according to the presentinvention, the unexposed DTR material was developed for 12 s at 24° C.in an aqueous alkaline solution as described in example 1 or 2.

The initiated diffusion transfer was allowed to continue for 18 s toform a silver layer, whereafter the material was rinsed with water at50° C.

One of the thus obtained metallic silver layers was used as such, onewas coated with a polyethylene layer (1 g/m²), a third one was coatedwith a novolac layer (1 g/m² Alnovol SPN452) and the last one was coatedwith a top layer of a copolymer of polyvinylbutyral, polyvinylalcoholand polyvinylacetate, esterified with trimetillitic acid anhydride (1g/m²).

Exposing the Heat-Sensitive Imaging Element

The 4 materials were all imaged with a Gerber C42T™ internal drumplatesetter at 12,000 rpm (367 m/s, pixel dwell time 0.032 μs) and 2540dpi. The power level of the laser in the image plane was 5.4 W.

After imaging, the plates were printed without any additional wettreatment.

Printing the Imagewise Exposed Element

The plates were printed on a Heidelberg GTO46 printing machine with aconventional ink (K+E) and a fountain solution of 5% G67c (commerciallyavailable from Agfa-Gevaert N.V.)+10% isopropanol.

The plate that did not get an extra coating layer after developing, didnot show any ink-uptake in the unexposed areas. The ones withpolyethylene and the one with the copolymer as a final coating, resultedin a good ink-uptake and no scumming, but the image was already damagedafter 25-50 prints. Finally the one with the novolac coating on top,showed a good ink-uptake, no scumming and a run-length>3000 prints.

Example 4

Preparation of the DTR Material

The DTR material was prepared as described in example 1 to 3.

Preparation of the Heat-Sensitive Imaging Element

To obtain a heat-sensitive imaging element according to the presentinvention, the unexposed DTR material was developed for 12 s at 24° C.in an aqueous alkaline solution as described in example 1 to 3.

The initial diffusion transfer was allowed to continue for 18 s to forma silver layer, whereafter the material was rinsed with water at 50° C.

One of the thus obtained metallic silver layers was used as such, threeother silver layers were coated with a novolac layer of respectively, 1,2 and 5 g/m² Alnovol SPN452.

Exposing the Heat-Sensitive Imaging Element

The 4 materials were all imaged with a Gerber C42T™ internal drumplatesetter at 12,000 rpm (367 m/s, pixel dwell time 0.032 μs) and 2540dpi. The power level of the laser in the image plane was 5.4 W.

After imaging, the plates were cleaned with a dry cotton pad andsubsequently printed.

Printing the Imagewise Exposed Element

The plate that did not get an extra coating layer after developing,showed strong ablation during imaging and no residues were left on theexposed parts. The plates with a novolac coating on top of 1 or 2 g/m²Alnovol SPN452, ablated as an easily removably yellow powder, while onthe one with a 5 g/m² novolac coating, still ablation residues werefound after cleaning.

The plates were printed on a Heidelberg GTO46 printing machine with aconventional ink (K+E) and a fountain solution of 5% G671c (commerciallyavailable from Agfa-Gevaert N.V.)+10% isopropanol.

The plate that did not get an extra coating layer after developing, didnot show any ink-uptake in the unexposed areas. All three layers with anovolac coating on top, showed a good ink-uptake and a runlength>3000prints.

What is claimed is:
 1. A heat-sensitive imaging material for makinglithographic printing plates comprising on a lithographic base, having ahydrophilic surface, a silver layer or titanium oxide layer and on topthereof an oleophobic polymeric layer having a thickness of less than 5μm wherein said polymeric layer comprises a polymer containing phenolicgroups.
 2. A heat-sensitive imaging material according to claim 1wherein said polymer containing phenolic groups is a phenolic resin or ahydroxyphenyl substituted polymer.
 3. A heat-sensitive imaging materialaccording to claim 1 wherein said silver layer of titanium oxide layerhas a thickness between 0.05 and 1.5 μm.
 4. A heat-sensitive imagingmaterial according to claim 1 wherein said lithographic base having ahydrophilic surface is a grained and anodized aluminum support.
 5. Aheat-sensitive imaging material according to claim 4 wherein thelithographic base having a hydrophilic surface is an anodized aluminumsupport which has been treated with a compound selected from the groupconsisting of polyvinylphosphonic acid, polyvinylmethylphosphonic acid,phosphoric acid esters of polyvinyl alcohol, polyvinylsulphonic acid,polyvinylbenzenesulphonic acid, sulphuric acid esters of polyvinylalcohol, and acetals of polyvinyl alcohols formed by reaction with asulphonated aliphatic aldehyde.
 6. A heat-sensitive imaging materialaccording to claim 1 wherein said lithographic base having a hydrophilicsurface comprises a plastic support provided with a hydrophilic bindercross-linked by means of a hydrolyzed tetraalkylorthosilicate.
 7. Amethod for making a lithographic printing plate comprising the step ofimage-wise exposing to actinic radiation a heat-sensitive elementcomprising on a lithographic base, having a hydrophilic surface, asilver layer or titanium oxide layer and on top thereof an oleophobicoleophilic polymeric layer having a thickness of less than 5 μm whereinsaid polymeric layer is not crosslinked and comprises a polymercontaining phenolic groups.
 8. A method for making multiple copies of anoriginal comprising the steps of: image-wise exposing to actinicradiation a heat-sensitive imaging material comprising on a lithographicbase, having a hydrophilic surface, a silver layer or titanium oxidelayer and on top thereof an oleophobic oleophilic polymeric layer havinga thickness of less than 5 μm wherein said polymeric layer is notcrosslinked and comprises a polymer containing phenolic groups, startingprinting without an additional wet treatment on the exposed imagingmaterial.
 9. A method according to claim 7 wherein said image-wiseexposure is carried out by an IR-laser.